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From: legg <legg@nospam.magma.ca>
Newsgroups: sci.electronics.repair
Subject: Re: Re (2): Voltage halver.
Date: Thu, 04 Apr 2024 13:11:35 -0400
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On 04 Apr 2024 09:37:02 -0700, peter@easthope.ca wrote:

>In article <vhct0j9ao9d75ad7dr47r4eha1t9knt5d2@4ax.com>, legg <legg@nospam.magma.ca> wrote: 
>> What does the microscope manufacturer supply for this function?
>
>The scope is a Zeiss OPMI 1.  Google finds many listings of various 
>models with various power supplies.  
>
>This scope was donated without the power supply; I can't say what the 
>original supply was but Zeiss built nice equipment capable of 
>continuous and reliable duty.
>
>> If you have a >6V source, simple 555-type buck regulators are 
>>  available with minimum output listed as 1.8V
>> 
>> https://www.aliexpress.com/item/1005006482720286.html?
>> 
>> These generally regulate the negative output terminal, unfiltered.
>> This would not be an issue for an isolated 6V incandescent lamp.
>
>Good idea.  Will see what Lee's Electronics has in stock. I expect to 
>have to limit the voltage somehow to prevent burning the bulb.
>
>Thanks,            ... P.

Using a lower voltage wall wart as a source is the easiest limiting 
method. Overvolting the bulb by 50% temporarily will not be 
catastrophic. If the wall wart is internally limited for power or 
current, then overvoltage is unlikely.

There may be issues with wall wart curremt limiting which is best
addressed with filtering (~ capacitive) between the two devices.

The 555 buck regulators are dumb, in themselves, which is often an 
advantage driving nonlinear (incandescent) loads. The unit with the 
large yellow PTC on-board is a good bet as load protection.

Just remeber that the 'common' rail in the system will be the +rail 
of the 555 buck output. The wall wart won't care - neither will 
the lamp. You're effectively driving the lamp with a negative supply.

The unfiltered output is a high frequency square wave with peaks 
equal to the wall wart's labelled voltage.

Filtering that output to DC would require an external 5A buck diode, 
a 5A choke and an electrolytic cap - effectively doubling the size, 
weight (and cost) of the regulator section, at no benefit to the lamp.

RL